ppl_04_e2

ID: 3658

Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com

C H A P T ER 1 4 :

Figure 15.1 Air masses acquire their distinctive characteristics from source regions.

Air masses, with distinctive characteristics, originate from different source regions around the globe. These air masses acquire the characteristics of the Earth’s surface underlying the areas over which they lie.

For example, if the surface of the Earth from which an air mass originates is cold and wet, then the overlying air, given enough time, will also become cold and wet. Conversely, if the source region is warm and dry, the corresponding air mass, with which the source region is related, will also be warm and dry.

In order for an air mass to take on the characteristics of the underlying surface, the air mass must remain over the source region for a considerable time. This is exactly what occurs in extensive areas of high pressure. In these high pressure areas, the winds are light, and the air mass itself remains stationary for a relatively long period of time.

The air

masses which affect

the world’s

weather originate from different source regions, and acquire the characteristics of the surface over which they lie.

The air

masses that affect the

British Isles

gain their initial characteristics in areas of high pressure, where an air mass may remain stationary for long periods.

Figure 15.3 Air masses which affect Europe and the British Isles: Polar Maritime, Tropical Maritime, Tropical Continental. Also Arctic air, Returning Polar Maritime.

An air mass will normally be either warmed or cooled as it is displaced, and this will dramatically change the nature of the air mass.

If an air mass moves towards a warmer area, for instance towards the Equator, it will be warmed from beneath. Its Environmental Lapse Rate (ELR) will then increase, and the air will become unstable favouring the formation of cumuliform clouds.

Conversely, if an air mass moves from its source region to a colder part of the globe, towards the poles, for instance, it will be cooled from below. In this case, its ELR will decrease, and the air will become stable. If cloud develops in this stable air, it will be low level stratiform cloud, perhaps forming from cumulous clouds which, unable to develop vertically, spread out horizontally, to become stratocumulus.

Below, we consider, in more detail, each of the air masses that we have identified so far, and give a description of how each air mass affects the weather over the British Isles.

P o l a r M a r i t i m e .

Polar Maritime air approaches the British Isles from the Northwest (see Figure 15.3), and is the air mass which affects Britain most frequently throughout the year. The source region for Polar Maritime air is the cold North Atlantic, but, when moving southwards, Polar Maritime air will pass over warmer areas of the Atlantic Ocean. This will cause the air mass to absorb moisture from the sea and become increasingly humid and unstable as it approaches the British land mass. As a result of the instability of this relatively moist airflow, cumulus and, occasionally, cumulonimbus clouds may develop. As the Polar Maritime air arrives over the British Isles, showers can be expected, accompanied by hail and thunder, under some circumstances.

Daytime flying conditions in Polar Maritime air are quite bumpy, due to turbulence caused by the air mass’s inherent instability. Visibility, however, is generally excellent outside areas of precipitation. Showers accompanying the Polar Maritime air usually die out overnight as the land cools, often leading to clear skies, temperature inversions, and sometimes, radiation fog, during the night and in the hours immediately after sunrise.

Polar maritime air is cold, moist and unstable.

Polar Continental

air is dry and cold,

originating from Siberia in the Winter and Spring months.

Arctic air is a specific type of

Polar Maritime air mass

which originates close to the North Pole and follows a long sea track before it reaches Northern Britain.

Tropical Maritime air

is warm and moist. As

it moves toward the British Isles, it is cooled from below and becomes stable, bringing advection fog and low cloud.

Tropical Continental air

is warm and dry and mainly

a summer phenomenon. Conditions will be dry, hot and cloudless, with poor visibility.

Returning Polar Maritime

air is an unstable moist

airflow, modified in its lower layers to become stable, because it initially tracks South and then turns back towards the British Isles on a northeasterly track.

P o l a r C o n t i n e n t a l .

The Polar Continental air mass affects Europe and the British Isles during the winter. Originating from Siberia or Northern Russia (see Figure 15.3), it is a stable, very dry and cold air mass which reaches Britain when easterly, or north-easterly winds become established. Polar continental air has little moisture, since it originates over an extensive land mass, and usually brings cloudless, but bitterly cold, weather to the British Isles.

If the Polar Continental air mass follows a sufficiently long track across a sea area such as the North Sea, there may have been enough warming of the air mass from beneath to cause convection to take place. Sufficient moisture may also have been picked up to cause some instability leading to snow showers in eastern coastal areas of Britain.

The visibility conditions accompanying the Polar Continental air mass are variable and dependent on the areas over which the air mass has travelled. If the Polar Continental air has originated in Scandinavia, the visibility is generally good; however, if the air mass has originated over the industrialised areas of Eastern Europe, visibility may be poor, due to atmospheric pollution.

A r c t i c .

Arctic air arrives in Britain as a strong, northerly airflow direct from the North Pole (see Figure 15.3). Arctic air quickly becomes moist and unstable as it moves southwards across the relatively warmer sea, bringing snow showers to Northern Scotland.

T r o p i c a l M a r i t i m e .

Tropical Maritime air originates in the mid-Atlantic from around the Azores, and approaches the British Isles from the South West. This air mass cools from below on its northward track which increases its stability. Since it is the lower layers which cool, only low cloud is produced, with accompanying poor visibility. Advection fog can also form, especially on windward coastlines. Inland conditions are often slightly better, but still relatively poor, with extensive low cloud. Inland in summer, however, there is usually enough heat from the Sun to lift, and occasionally break, the cloud into shallow cumulus cloud.

T r o p i c a l C o n t i n e n t a l .

Tropical Continental air approaches the British Isles from the South East. This air mass mainly affects Britain in the summer, and is responsible for the highest temperatures. Tropical Continental air is dry, usually with cloudless skies, although hazy conditions often predominate, with moderate to poor visibility.

R e t u r n i n g P o l a r M a r i t i m e .

If Polar Maritime air undergoes a long southerly journey, and then swings towards the British Isles on a north-easterly track, it is called Returning Polar Maritime air. (See Figure 15.3.) As its initial track was southwards, Returning Polar Maritime air becomes warmer and more unstable than the classic Polar Maritime air mass, and so contains a significant amount of moisture gathered on its passage over the

Atlantic Ocean.

However, as Returning Polar Maritime air swings North East towards the British Isles, it becomes stable in its lower layers as it moves to a relatively cooler region, and so cools from below. Meanwhile the middle and upper layers remain unstable. Cloud